REITs are traded on the market, (pretty much) just like a stock. So you don't have to worry about liquidity being too much of a problem. One example of a REIT is [1].
However, if you really just want to park money for 20 years, you probably want a target year fund like [2] which will have higher risk now (stocks, REITs, etc), and rebalance to lower risk (Bonds) as it gets closer to 2040. It's literally the definition of set it and forget it.
edit: I should add, I'm not your financial advisor, and I don't have a fiduciary duty. You should consult with a financial advisor (who has a fiduciary duty) before making any decisions about investments. However, target year funds are frequently (one of) the best decisions for many people.
However, if you really just want to park money for 20 years, you probably want a target year fund like [2] which will have higher risk now (stocks, REITs, etc), and rebalance to lower risk (Bonds) as it gets closer to 2040. It's literally the definition of set it and forget it.
[1] https://personal.vanguard.com/us/funds/snapshot?FundId=0123&...
[2] https://personal.vanguard.com/us/funds/snapshot?FundId=0696&...
edit: I should add, I'm not your financial advisor, and I don't have a fiduciary duty. You should consult with a financial advisor (who has a fiduciary duty) before making any decisions about investments. However, target year funds are frequently (one of) the best decisions for many people.